Table 13 - Excerpt from "Vitotrans 353", type PBMA/PZMA datasheet
Heating wa-
ter tempera-
ture in the
heating wa-
ter buffer
cylinder
Set DHW
tempera-
ture
Max. draw-
off rate
from
Vitotrans
353
Transfer
output
Required
heating
water buf-
fer cylin-
der volume
per l of
DHW
At 10 °C cold water inlet temperature:
Max. draw-off rate at the mixing valve at
Return tem-
perature to
the heating
water buffer
cylinder
40 °C 45 °C 50 °C 55 °C
in °C in °C in l/min in kW in l in l/min in l/min in l/min in l/min in °C
40 60 125 0.4 — — — — 14
45 60 146 0.5 70 — — — 15
70
50 52 144 0.8 68 58 — — 17
55 44 137 0.9 65 56 49 — 20
Ó
60 37 127 1.1 60 52 45 40 23
Calculating the required buffer volume
To provide the energy required for DHW heating, a freshwater mod-
ule is normally connected to a heating water buffer cylinder. The
heating water buffer cylinder volume depends on the DHW demand
of the installation, the storage temperature in the heating water buf-
fer cylinder and the user behaviour.
The following applies:
V
P
=
´ x t x (T
P
/T
WW
) x s
N
V
P
= Required minimum volume of the heating water buf-
fer cylinder
´
= Calculated peak flow rate of the freshwater module
t = Length of time for which the peak flow rate is re-
quired. The value can be based on e.g. the time tak-
en to fill the bath, information provided by the user,
or the standard value from DIN 4708 (10 min).
(T
P
/T
WW
) = for the temperature spread between heating water
buffer cylinder and DHW:
0.5 = when temperature spread is large
(e.g. 90/45 °C)
0.7 = when temperature spread is medium
(e.g. 70/45 °C)
1.0 = when temperature spread is small
(e.g. 55/45 °C)
s
N
= Safety factor for consideration of user behaviour:
1 = normal draw-off pauses
2 = short draw-off pauses
3 ... 4 = very short draw-off pauses
Example:
A buffer cylinder is to be selected for the detached house in the
example on page 22 (chapter "Calculating the DHW demand").
The peak flow rate is 29 l/min.
The future system user has indicated that he "enjoys long showers".
He has indicated a demand duration of 15 min.
For reasons of energy efficiency, the storage temperature in the buf-
fer cylinder should be no more than 70 °C.
The draw-off temperature is 60 °C.
This results in a low temperature spread of 70/60 °C, giving a correc-
tion factor of 1.
As the future system user has indicated that he "enjoys long show-
ers", short draw-off pauses have been assumed. Therefore, the
safety factor s
N
is 2.
The minimum buffer volume V
P
is therefore:
V
P
=
´ x t x (T
P
/T
WW
) x s
N
= 29 l/min x 15 min x 1 x 2
= 870 l
According to the datasheet, a Vitocell 100-E with a volume of 950 l is
selected.
Sizing
(cont.)
DHW heating
VIESMANN
23
5414646
4
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